Sunday, March 26, 2017

Can birds perceive rhythmic patterns?

The specific question whether animals can detect regularity in a stimulus and synchronize their own behavior to arbitrary rhythmic patterns got sudden attention with the discovery of Snowball, a sulphur-crested cockatoo that could synchronize head and body movements with the beat in several popular songs (see earlier entry). Parrots, such as Snowball, are vocal learners and vocal learning is associated with evolutionary modifications to the basal ganglia, which play a key role in mediating a link between auditory input and motor output during learning. As such linkage between auditory and motor areas in the brain is also required for beat induction, Patel suggested that only vocal learning species might be able to show beat induction.
However, further studies have shown the picture to be more complicated (see earlier entry) and this calls for a re-examination of the link between vocal learning and beat perception and induction. While zebra finches (vocal learners) are able to discriminate a regular isochronous from an irregular stimulus (Van der Aa et al., 2015), this discrimination was strongly reduced with tempo transformations (changing rate, but not the regularity of the stimulus). Zebra finches seem to attend strongly to specific local features of the individual stimuli (e.g. the exact duration of time intervals) rather than the overall regularity of the stimuli, which was the main feature human listeners attended to (Van der Aa et al., 2015).

Figure 3 from Ten Cate et al. (2016)

In a recent paper (Ten Cate et al., 2016) we review the available experimental evidence for the perception of regularity and rhythms by birds, like the ability to distinguish regular from irregular stimuli over tempo transformations and report data from new experiments. While some species show a limited ability to detect regularity, most evidence suggests that birds attend primarily to absolute and not relative timing of patterns and to local features of stimuli. We conclude that, apart from some large parrot species, there is limited evidence for beat and regularity perception among birds and that the link to vocal learning is unclear. We next report experiments in which zebra finches and budgerigars (both vocal learners) were first trained to distinguish a regular from an irregular pattern of beats and then tested on various tempo transformations of these stimuli. The results showed that both species reduced the discrimination after tempo transformations. This suggests that, as was found in earlier studies, they attended mainly to local temporal features of the stimuli, and not to their overall regularity. However, some individuals of both species showed an additional sensitivity to the more global pattern if some local features were left unchanged. Altogether our study indicates both between and within species variation, in which birds attend to a mixture of local and global rhythmic features.